A sensor is a device that changes a physical quantity to another physical quantity in response to a physical stimulation.
There are typically two kinds of sensors;
Contacting sensor that perceives the existence of an object by contacting the object; and
Non-contacting sensor that perceives the existence of an object without contacting the object.
The contacting sensor is used wider than the non-contacting sensor. For one example of the contacting sensors, a limit switch is often used in a place where the safety needs to be secured, for example, to protect from explosion, and it is not sensitive to an effect of magnetic field.
However, it was difficult to install a conventional contacting sensor to some part of the limited device that is narrower than the limit switch space.
Therefore, the conventional contacting sensors have limitations to install to the various featured devices.